Gee, a "mere" 20 hours in combat. I'm sure I do not know how you distinguish "combat" hours from "waiting to get shot at" hours. Say loss rates "in combat" are 1 per hundred hours. That means you have about a one in five chance of being killed during those mere twenty hours. How may people would sign up for a job like that.
In World War II bomber crews were rotated home after 25 missions. I'm not sure Vietnam, with radar guided missiles and a period when MiGs actually out performed U.S. fighters was a whole lot less stressful.
This discounts the hazards not related to combat, like the chance of getting killed in a fire on the deck or when landing or taking off from a ship at sea.
The Aircraft carriers were just off the coast, it took maybe five or less minutes to be in the combat zone after takeoff. Many bomb runs in VietNam involved a small bomb load against a single target. Tim in combat is no measure of risk.